Absolute bioavailability in man of N-acetylprocainamide determined by a novel stable isotope method

Absorption of a single oral dose of N-acetylprocainamide (NAPA) was studied in 3 normal subjects. Approximately 85% of the oral dose was absorbed and peak plasma NAPA concentrations were reached in 45 to 90 min. In 2 subjects, NAPA was absorbed at a fast initial rate, then more slowly, prolonging the apparent elimination phase half-life. Absolute bioavailability was determined by a new stable isotope method that entailed intravenous injection of NAPA 13C at the same time that an unlabeled NAPA capsule was given orally. Plasma levels and urine excretion of both compounds were determined by mass fragmentography. Bioavailability was assessed by deconvoluting the plasma level vs time curves resulting from intravenous and oral drug administration, and also by comparing the relative percentage of NAPA and NAPA-13C excreted unchanged in the 24 hr after simultaneous administration.